Selective Catalytic Reduction (SCR) catalyst is a type of catalytic converter which purifies NOx contained in exhaust gas. When reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbons (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced by the SCR catalyst through an oxidation-reduction reaction with the reducing agents.
When a lean NOx trap (LNT) and the SCR catalyst are used together, the SCR catalyst may be coated on a diesel particulate filter due to space restriction. A selective catalytic reduction catalyst on diesel particulate filter (SDPF) absorbs particulate matter contained the exhaust gas and eliminates NOx contained in the exhaust gas.
Generally, an NOx sensor is installed at a front end and a rear end of the SCR for measuring NOx purification efficiency of an SCR system. Difference between values measured at the front end and the rear end of the SCR is amount of NOx purification, and means efficiency of the SCR system. However, this NOx sensor installed at the vehicle also responds to NH3, and detects NH3 as NOx.
The NH3 is sufficiently supplied to a part of section of the SCR system, and the NH3 is insufficiently supplied to the remains. In a section where the NH3 is sufficiently supplied, the NH3 is absorbed and slipped when it becomes over absorption capacity. If a urea injection is stopped to decrease NH3 slip in the section where the NH3 is sufficiently supplied, NOx purification in a section where the NH3 is insufficiently supplied is prevented, thereby amount of NOx exhaust of a tail pipe increases.
When the NH3 slip is generated in the part of the section of the SCR, the NOx sensor at the rear end of the SCR misrecognizes the slipped NH3 as unpurified NOx to feedback control so as to increase the amount of urea supply. In this case, the NH3 slip increases continuously, and NOx purification efficiency measured by the NOx sensors of the front and rear ends of the SCR becomes deteriorated. Accordingly, it is impossible to control correction of a urea injection and monitor an efficiency of the SCR. In other words, although the SCR still maintains a good purification efficiency, but a purification efficiency measured using an NOx sensor may be calculated to be deteriorated. Accordingly, in view of OBD (On-Board Diagnotics; exhaust gas self diagnosis), a normal SCR may be diagnosed as a deficient SCR.
Particularly, when an NH3 is detected during an overrun, detection range is limited and detection in various driving conditions is not accurate. Further, contributiveness of NOx and NH3 may not be recognized in the NOx purification efficiency.
Further, it is not recognized whether the SCR system calculating the NOx purification efficiency to be low is really out of order to have a low NOx purification performance or the NOx purification performance is maintained to be good but a distortion of a sensor value is happened by an NH3 slip.
Accordingly, a normal component may be diagnosed as a disabled component because an influence degree of an NOx purification efficiency by an NH3 slip is not determined, and in this case, the entire SCR components has to be changed to make a generated cost high.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.